1. Field of the Invention
The present invention relates to a pop sound reduction circuit which is applied to a voice output device constituted by semiconductor integrated circuits, and more particularly to a pop sound reduction circuit which reduces pop sound generated at the time when power is turned on/off and a voice output amplification device comprising the same.
2. Description of the Related Art
In terms of a voice output amplification device having a Single Ended Push-Pull (SEPP) circuit structure using a conventional switching system, typified a D-class amplifier, a power supply from one electric power source has been considered to be advantageous from the viewpoint of a circuit scale and cost. In the voice output amplification device having the SEPP circuit structure using the switching system, to which power is supplied from one electric power source, a pulse width modulation amplifier performs a switching operation at a constant frequency and a constant duty ratio (usually 50%) so that an output reference potential during a normal operation becomes equal to a predetermined DC value (usually about VCC/2). In this case, the output reference potential obtained by the switching operation by the pulse width modulation amplifier at the time when the power source is turned on reaches smoothly from a potential of a GND level to a predetermined output DC potential during the normal operation. During this period of time, noises having a frequency of an audio band should not be generated. However, it is actually difficult to obtain a transitional operation in which the output reference potential reaches from the GND potential level to the desired DC potential, by controlling an output switching frequency and a duty ratio. In the vicinity of the lowest operation voltage of the circuit itself, the switching frequency itself passes through the audio band, and thus the switching frequency may generate noises called pop sound. On the other hand, when the electric power source is turned off, the output reference potential decreases smoothly from the value during the normal operation to the GND level when the electric power source is turned off. During this period of time, no generation of noises having an audio frequency band is desired, and it is desirable that the circuit is stopped in a state where unnecessary noises occur after turning-off of the electric power source. However, it is difficult to control the output reference potential similarly to the time when the electric power source is turned on, and there is a possibility of occurrence of pop sound due to transitional variations of the switching frequency in the vicinity of the lowest operation voltage of the circuit itself.
To solve such a problem, in the voice output amplification device having the SEPP circuit structure using the switching system, to which power is supplied from one electric power source, a countermeasure to improve the level of the pop sound by canceling the voltage variation of GND by making the output a BLT structure and by making the power supplied from two electric power systems. However, when the BTL structure is adopted, the number of amplifiers per one channel is doubled, and when the voice output amplification device is powered from two electric power sources, the structure of the power sources become complex. Therefore, the voice output amplification device encounters a problem of high cost.